Shanks is one of the Four Emperors of the Sea and the captain of the Red Hair Pirates. Before the fall of Kaido and Big Mom, Shanks was the youngest of the Four Emperors, which is a testament to his power. Out of all the powerful pirates, Shanks is the only one who doesn't seem to have a devil fruit, so he relies on his mastery of Haki to beat his opponents.
Towards the end of the Wano Country Arc, Shanks was seen scaring one of the Navy admirals with his burst of Conqueror's Haki. While it is expected of an emperor to be strong it is a huge feat considering that Shanks was nowhere near Greenbull's location. Currently, Shanks is arguably the strongest character in One Piece, which means there are only a handful of characters that can challenge him.
7 Akainu
Akainu is the Fleet Admiral of the Marines. Endowed with the power of the Magu Magu no Mi, he is the strongest Marine. His devil fruit allows him to create and control magma at will. Many consider his devil fruit to have the highest offensive power in the logia class.
Akainu is known for his tenacity and ruthlessness. During the Summit War, he was chasing pirates even after he had successfully killed Portgas D. Ace. The only person who was able to overwhelm him was Whitebeard. Shanks and Akainu have clashed once, but that was a long time ago. Since then, Akainu has gained promotion and arguably become much stronger. A rematch between the two would definitely be interesting.
6 Luffy
Monkey D. Luffy is the main protagonist of the series. Since starting his journey to become the Pirate King, Luffy has undergone several ordeals that have made him stronger. The biggest test of Luffy's strength came in the Wano Country arc, where Luffy was up against Kaido, who held the title of the "Strongest Creature in the World."
The fight against Kaido helped Luffy to awaken his devil fruit and use it to the fullest. In addition to this, Luffy also learned how to coat his body in Conqueror's Haki. This combination of his devil fruit and Haki allowed Luffy to damage Kaido's indestructible body. Shanks knows that Luffy has come a long way since they last met, and if they were to fight, it would definitely be one of the best, if not the best fight in the series.
5 Blackbeard
Marshall D. Teach, also known as Blackbeard, is one of the Four Emperors of the Sea. Blackbeard rose to prominence after the Summit War. In a short period, he managed to become an Emperor and gain a ridiculously strong crew. Due to his special body, Blackbeard was able to consume a second devil fruit, which was the Gura Gura no Mi.
This devil fruit allows him to create quakes that can potentially destroy an entire island. To make matters worse, Blackbeard's Yami Yami no Mi enables him to engulf anything inside a pit of darkness, and he can even negate the devil fruit powers of his opponents. Blackbeard has also been shown using two types of Haki, but his proficiency remains to be seen. It is clear that Shanks and Blackbeard have a history, so if they fight again, one of them is bound to die.
4 Dragon
Monkey D. Dragon is the leader of the Revolutionary Army. The goal of his organization is to topple the evil World Government and liberate the world. Dragon has appeared only a handful of times in the series, and he has yet to show off his power. However, judging by the fact that he has complete authority over other powerful characters such as Sabo, Koala, Karasu, Morley, and Belo Betty, Dragon must at least be more powerful than them.
For context, Sabo was easily able to fend off Admiral Fujitora in Dressrosa which implies that Dragon must be at least on the same level as an Emperor. The fight between Dragon and Shanks is highly unlikely to occur, but if it were to happen the fans can definitely expect a spectacle.
3 Imu
Imu is the real leader of the World Government. They made their debut in the Levely Arc, where the Five Elders could be seen bowing before them. Presently, there is very limited information available about them, but that will change in the final saga.
As the leader of the World Government, Imu seems to have access to one of the Ancient Weapons that can decimate an entire island within seconds. Suffice it to say, Imu probably has a devil fruit as well, and fans can expect it to be very powerful. The confrontation between Shanks and Imu might never happen, but the latter is one of the few characters who can give the Emperor a challenge.
2 Dracule Mihawk
Dracule Mihawk holds the title of the "Strongest Swordsman in the World." He is currently a member of the Cross Guild, an organization that is actively hunting down Marines. Before Mihawk was offered a place within the Shichibukai, he used to be known as the "Navy Hunter."
Mihawk also used to be rivals with Shanks and according to Whitebeard, the battles between the two were nothing short of legendary. After Shanks lost his arm, Mihawk stopped fighting him. If they decide to fight again in the future, it would settle the age-long debate about who is stronger between the two.
1 Trafalgar Law
Trafalgar Law is a member of the Worst Generation and the captain of the Heart Pirate. Law possesses the Ope Ope no Mi, a paramecia-type devil, which is considered by many to be the "Ultimate Devil Fruit." After eating his devil fruit at a very young age, Law successfully managed to master every ability of the devil fruit.
While fighting Big Mom, Law demonstrated the power of his devil fruit's awakening, and he used it to defeat her, albeit he did need some help from Eustass Kid. In the recent One Piece chapters, Law was seen fighting against Blackbeard on equal footing, which is a huge feat considering the caliber of his opponent.